Hydrogen Uptake Enhancement and Accelerated Hydrogen Re-embrittlement of Cd-plated AISI 4340 Steel Bolts Coupled with IN
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im of this communication is to highlight a significant risk of hydrogen re-embrittlement (HRE) for anodically coated high strength steels (HSS) coupled with more cathodic (nobler) metals and in the presence of crevices. HRE is an embrittlement phenomenon that interests anodically electroplated steels.[1–3] HRE is different from typical hydrogen embrittlement (HE)[4–7] because the latter is usually caused by direct hydrogen absorption during component production (steel processing, acidic pickling, plating, and insufficient degassing), whereas the former is a consequence of hydrogen uptake during the operative life of the component when the coating undergoes corrosion in service.[2] Both in HRE and in HE, once atomic hydrogen enters in the metallic matrix, it diffuses toward regions of high stress states, and depending on the aqueous environment GIAN FRANCO LOVICU, Research Assistant, COSTANZA COLOMBO, Graduated Student, MASSIMO DE SANCTIS, Associate Professor, and RENZO VALENTINI, Assistant Professor, are with the Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali, Universita` di Pisa, 56126 Pisa, Italy. Contact e-mail: [email protected] Manuscript submitted January 26, 2011. Article published online September 15, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
corrosivity and on the material hydrogen embrittlement susceptibility, the steel component could be embrittled and suffer for brittle cracking. This study was carried out because of the observed failure of a Cd-coated AISI 4340 bolt coupled with a IN718 nut. The use of cadmium coatings is likely to be reduced in the future for environmental reasons.[8–11] However, the phenomenon discussed here may take place also with other anodically coated steel components. Concerning hydrogen-induced cracking, HRE is the most dangerous attack for anodically coated HSS components because the protective surface coating may be damaged, especially in bolt thread regions, where it is likely to be mechanically scratched during assembling. In this case, a Cd-coated AISI 4340 bolt, immersed in artificial sea water, failed in a relative short time after having been assembled to an uncoated IN718 nut. The crack started on the thread flank of the bolt and resulted in a brittle intergranular fracture (see Figure 1), which is typical of hydrogen-induced cracking. The failure was unexpected because the coupling with a more cathodic IN718 (in passive state) was expected to shift the mixed corrosion potential of metallic assembly toward more electropositive potentials, thus decreasing the amount of hydrogen evolution onto the steel surface. In this respect, the presence of a crevice could have played some effect, and it should be taken into account. To measure the hydrogen uptake of steel in different conditions, a modified Devanathan–Stachurski[12] permeation apparatus was used. An AISI 4340 steel membrane (32 mm in diameter, 0.75 mm in thickness), Cd-electroplated on one side, was chosen as a reference specimen. Cadmium was removed only from a central circular a
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